I have a shaded pole blower motor with speed control that doesn't maintain constant speed beyond a certain setting. With the speed control plugged in to 115v and no load I can measure the voltage output varying constantly from 75v to 90v. When I plug two spare blowers into the controller the speed can be varied constantly just as expected. But when I plug my fireplace blower in and turn it up halfway, I measure the voltage under load at 90v and it maintains constant speed for a few minutes, then it increases to almost max speed while the voltage remains constant. Then I can vary the voltage from 90 to 115 and back and the speed just stays the same. If I drop it way down to about 85v it will eventually slow down to the expected speed. But if I move it back up to 90v it will eventually ramp up to max speed again. Any insights as to what is going on here and if it can be fixed? Apparently it is a problem with the motor and not the controller.